A new study led by a Brown University biologist reveals that a species of marine snail uses a unique method of agriculture. According to the study in the Proceedings of the National Academy of Sciences, the snail cultivates the growth of fungus, its preferred food, on live marsh grass, leading to a fungal infestation that suppresses marsh grass production. Fungal farming was previously thought to occur only in terrestrial insects.

PROVIDENCE, R.I. — A startling mutual-aid society
is linking fungus and snails in marine ecosystems, according to a study led by a
Brown University biologist. The study presents the first evidence that a species
of marine snail engages in a previously undemonstrated form of food acquisition
and ecological control by initiating and encouraging the growth of fungi, its
preferred food, on live marsh grass. Infestation by fungi greatly slows the
growth of the grass.

The farmer in the shellAlong the southern U.S. seashore, the snail, Littoraria irrorata, contributes to the growth of fungi – its favorite food – on marsh grasses. Researchers call that process “fungal farming.” Laboratory studies have shown that snails are more robust when fed a combination of marsh grass and fungi rather than grass alone.

In surveys conducted along 2,000 kilometers of salt marshes on
the southern U.S. shoreline, the researchers observed that the snail,
Littoraria irrorata, actively grazes a live salt-marsh cordgrass. As the
snail crawls along the grass surface, it scrapes grass tissue with its band of
saw-like teeth and creates longitudinal cuts in leaf surfaces, making a much
larger meal possible. While it travels, the snail also deposits feces laden with
fungal spores and nutrients into the sensitive inner-tissue of the leaf,
effectively stimulating and fertilizing fungal crops.

The result of snail grazing on marsh grass surface is an
infestation of fungi, a major diet component for the snail, and the slowing of
marsh grass growth.

“In its manner of manipulating fungi, the snail is
conducting a low-level form of food production,” said lead scientist Brian
Silliman, an ecology and evolutionary biology doctoral candidate at Brown
University. “This is fungal farming in a completely new group (phylum) of
organisms and the first demonstration in the marine environment.”

This method of agriculture – called
“fungiculture” – was thought to occur only in three distinct
insect lineages, including certain ants, termites and beetles. The development
of fungus-growing behavior has enabled these insects to rise to major ecological
importance in land communities where they can strongly affect ecosystem process
and community structure through their farming activities.

The new study, which appears in the current Proceedings of
the National Academies of Sciences, establishes the mechanism by which the
snail, among the most abundant grazers found in southern salt marshes, is able
to affect its ecosystem. Silliman authored the study with Steven Newell of the
University of Georgia Marine Institute.

Southern salt marshes have been among the most productive
grasslands in the world. In addition, they serve important ecological functions,
including tempering coastal flooding, filtering mainland run-off, and acting as
nurseries for commercially important fish and other species.

Earlier research by Silliman and others observed that the snail,
long thought to eat only dead or dying plant materials, also grazed live
salt-marsh cordgrass. A 2002 collaboration with another Brown researcher
indicated that the snail participates in a top-down “trophic
cascade” in which the control of grazers (snails) by predators (crabs) is
a major determinant of plant growth in the marine environment.

It is the grass that suffersFungi transmitted by snail feces thrive on the sensitive inner layers of leaves which have been exposed by the saw-like teeth of grazing snails. While snails and fungi thrive on that arrangement, damage done to the plants may be a significant factor in limiting the growth of marsh grasses.

The earlier findings suggested that the over-harvesting of snail
predators may be an important factor contributing to the massive die-off of salt
marshes across the southeastern United States in recent years. According to
Silliman, several other theories have been advanced to help explain marsh
die-off, including drought and the dispersion of a fungal pathogen.

By suggesting a synergy between fungus and snail that leads to
increased growth for both and diminished growth for the host marsh grass, the
new study may add to discussion of the role of fungi in this environmental
change.

In the laboratory, the researchers observed that snails fed
fungus and marsh grass were more robust than snails fed marsh grass leaves
alone. In addition, field experiments determined that the fungus was more
abundant in the snail-maintained wounds than on the leaf surfaces. Fungal
removal experiments indicated that fungal invasion of the leaves was a greater
determinant of grass deterioration than was snail grazing.

The snail is the first gastropod, or mollusk, shown to exhibit
fungal-farming behavior, Silliman said. However, the researchers note that given
the biological simplicity of this low-level fungal production, fungal farming
may be more geographically and phylogenetically widespread than presently
envisioned, especially in systems where fungal spores are abundant, grazers can
manipulate fungus growing media and fungus is a major diet of consumers.

The National Science Foundation, Environmental Protection Agency
(Science To Achieve Results fellowship), and the National Oceanic and
Atmospheric Association Narragansett Bay Fellowship funded the study.